2.5. Conclusions

In this chapter a brief overview and evaluation of the emissions scenarios
is presented. Much of the quantitative analysis of the emissions scenarios is
based on the unique scenario database developed for SRES. This database, its
structure, and the process of assembling the data that it now contains are described.
The database provides a valuable overview of the various emissions modeling
approaches and scenarios in the literature. It is the most comprehensive ESD
available, and it can be accessed through a website (www-cger.nies.go.jp/cger-e/db/ipcc.html).

The SRES database represents the basis for evaluations of emissions scenarios,
their main driving forces, and their uncertainty ranges. The CO2 emissions trajectories
of the scenarios in the SRES database are presented and their distribution and
the associated sample statistics assessed. In the same way, the main driving
forces of future emissions - population, economic development and energy consumption
- are analyzed. Finally, the possible relationships among these driving forces
for the collection of emissions scenarios in the database are considered.

Future levels of CO2 emissions from the energy sector are a function of population,
gross world product, the structure and efficiency of the economy, and the costs
and availability of different sources of energy.

The factor range for population increases by 2100 across the scenarios is between
1.2 and 3.3 times 1990 levels (see Figure 2-4).
This is the smallest factor increase of all the emissions driving forces. This
probably reflects a relatively high consensus among demographers as to future
population growth. However, this observation is based on a relatively small
number of reported population projections in the database (46). The range in
projected gross world product values is between 3.2 and 35 times the 1990 levels
(see Figure 2-5). The range of the factor increase
of primary energy consumption is from 0.9 to 10 times 1990 levels (see Figure
2-8). For energy intensity the range in 2100 is from 9.3 MJ/US$ down to
2.3 MJ/US$.

The range in carbon intensity of primary energy is the widest range of all
the driving forces considered here. It varies from 0.025 to 1.1 times 1990 levels
(about a factor of 45) in the year 2100 (see Figure
2-11). Emissions trajectories are extremely sensitive to a number of driving
forces, which include including population growth, economic growth, and energy
intensity improvement. Variation in carbon intensity is the main indicator of
the wide variation in energy-related CO2 . However, it is important to recognize
that this is a result of the inputs, assumptions, methods, and types of models
used to calculate the scenarios.

The findings of the analysis of the scenarios in the literature suggest the
following general conclusions:

the ranges over which the driving forces vary are large, which contributes
to the wide range of future emissions;

of these driving forces, gross world product and population are often exogenous,
while carbon and energy intensity are not;

the frequency distributions of driving forces and emissions are asymmetric,
with long tails and often more than one peak;

for many driving forces the median and mean are closer to the minimum than
to the maximum;

lack of information as to whether climate intervention policies and measures
were included in the scenarios, for about 40 scenarios, means that it is not
possible to distinguish between intervention and non-intervention scenarios;

it is hoped that IPCC or another international institution will, in the
future, maintain this or a similar database so as to assure a continuity of
knowledge and scientific progress in GHG emissions scenarios (an equivalent
database to document narrative and other qualitative scenarios would also
be very useful for future climate-change assessments); and

it would be useful in the future to evaluate the consistency of the driving
forces and results of all scenarios included in the scenario database. In
addition, it would be helpful to extend the database to include land-use change
data where currently only land-use emissions are presented. As noted above,
only a small percentage of scenarios contain information on land use.